Your baby's development in the third trimester
Weeks 28 to 40. The trimester of finishing touches — a brain that folds into complexity, fat that doubles body weight, lungs that practise breathing, and a baby who already knows your voice. Here is what is happening.
🌿 Open the Pregnancy Week Guide in WiseMama — freeThe third trimester at a glance
The first trimester builds the architecture. The second trimester opens the senses and establishes the person. The third trimester is about completion — the systems that were constructed in earlier weeks are refined, stress-tested, and prepared for the demands of independent life outside the womb.
It is also the trimester of the most dramatic visible change. At 28 weeks your baby weighs roughly one kilogram. At 40 weeks, around 3.4 kilograms. That near-quadrupling of weight in 12 weeks is driven by fat accumulation on a scale not seen at any other stage of development — and it matters not just aesthetically but physiologically, for temperature regulation, for energy reserves in the first days of life, and for the cushioning that protects vital organs during birth.
The brain: folding into complexity
The most structurally dramatic development of the third trimester happens in the brain. The cerebral cortex, which has been growing throughout pregnancy, begins the second trimester relatively smooth. By the end of the third trimester it has transformed into the deeply folded tissue that is the hallmark of the fully developed human brain — and this transformation happens almost entirely in the weeks from 28 to 40.
Gyrification: why the brain folds
The folds of the brain — gyri (the ridges) and sulci (the grooves between them) — are not arbitrary. They are the solution to a geometric problem: the human cerebral cortex, if unfolded, has a surface area of approximately 2,500 cm² — roughly the size of a pillowcase. Fitting that surface area into a skull that must pass through the birth canal requires extensive folding.
But the folding is not only structural. Each fold brings different regions of the cortex into proximity with each other, shortening the distance that neural signals must travel between connected areas. The pattern of gyrification is therefore intimately linked to the efficiency of neural communication — and disruptions to the folding process, from premature birth or certain infections, are associated with cognitive and developmental differences.
From week 28, the brain adds approximately 250,000 neurons per minute — a rate that continues until around 34 weeks before slowing as the focus shifts from producing new cells to connecting the ones already present. The myelination of neural pathways — the process of coating nerve fibres in the insulating myelin sheath that dramatically speeds electrical signals — continues not just through the third trimester but through the first years of life.
Sleep cycles and dreaming
By week 27 your baby was experiencing REM sleep. By week 28 this is well-established, with clear alternating cycles of active (REM) and quiet sleep. The significance of this is not sentimental: REM sleep in foetal development is thought to play an active role in neural maturation, stimulating synaptic development in ways that do not happen during quiet sleep. Your baby is not simply resting during this period — the brain is working.
Fat: the trimester's most visible work
Subcutaneous fat — the fat layer directly beneath the skin — is almost entirely absent before 28 weeks. The thin, translucent appearance of very premature babies reflects this: the fat that gives term newborns their characteristic rounded appearance takes all of the third trimester to accumulate.
This fat serves several distinct purposes, each critical for the newborn period:
- Thermoregulation — newborns cannot shiver to generate heat. The fat layer provides insulation against heat loss and a metabolic reserve that can be mobilised to generate warmth in the first hours of life outside the womb
- Energy reserve — in the first days of life, before feeding is established, the fat stores provide a buffer against hypoglycaemia (low blood sugar). Premature babies and growth-restricted babies with reduced fat stores are at higher risk of neonatal hypoglycaemia
- Brown adipose tissue (BAT) — a specialised thermogenic fat found particularly around the neck, shoulders, and kidneys, which can generate heat through a process called non-shivering thermogenesis. Unlike adult fat, brown adipose tissue burns calories to produce heat rather than storing them — a mechanism specific to newborns
The rate of fat accumulation peaks between weeks 32 and 36 at around 200–250g per week — the fastest growth rate at any point in the pregnancy.
The lungs: the last system to mature
The lungs are the last major organ system to complete their development, and the pace of their maturation is one of the most consequential aspects of the third trimester. This is why every additional week of gestation matters, and why elective birth before 39 weeks is avoided where possible.
Surfactant production
Surfactant — the substance that lines the inner surface of the alveoli and prevents them from collapsing after each breath — began to be produced in small quantities from around week 24. But production increases significantly through the third trimester, reaching clinically adequate levels by around 34–36 weeks in most pregnancies. Without sufficient surfactant, each breath requires enormous effort and the air sacs collapse after expiration — a condition called respiratory distress syndrome, which is the primary cause of morbidity in premature infants.
Alveolar development
Beyond surfactant, the alveoli themselves — the tiny air sacs where oxygen and carbon dioxide are exchanged — continue to multiply through the third trimester and beyond. A full-term newborn has approximately 50 million alveoli; an adult has around 300 million. The majority of that growth happens in the first two years of life, but the baseline established by full gestation matters — babies born at 37–38 weeks have fewer alveoli than those born at 39–40 weeks, and this difference has measurable effects on respiratory health in early childhood.
Breathing practice
From week 14, your baby has been making rhythmic breathing movements — drawing amniotic fluid into the developing lungs in practice runs for the real thing. In the third trimester these movements become more sustained and coordinated. Occasionally they cause hiccupping, which you may feel as regular, rhythmic jolts quite different from kicks and rolls.
Antibody transfer: the placenta's last great gift
Newborns enter the world with an immune system that is structurally complete but immunologically inexperienced — they have not yet encountered the pathogens they will meet in the outside world. The mechanism that bridges this gap is one of the most elegant in human biology: the transfer of maternal antibodies across the placenta.
From around week 28, immunoglobulin G (IgG) antibodies — the class responsible for long-term immunity — cross the placenta in increasing quantities. These antibodies represent your accumulated immune history: protection against every pathogen your immune system has ever encountered, every infection fought, every vaccine received. By term, a full-term baby has antibody levels comparable to or exceeding their mother's for many common infections.
The transfer rate increases significantly in the final weeks of pregnancy. A baby born at 32 weeks receives substantially fewer maternal antibodies than one born at 40 weeks — one of the reasons premature infants are more vulnerable to infection in the first months of life. This is also why vaccines received during pregnancy — whooping cough in particular, recommended at weeks 16–32 in the UK — directly protect the newborn through this transfer mechanism.
Colostrum and continued protection
The protection delivered by placental antibody transfer is continued after birth through colostrum — the first breast milk, which is exceptionally rich in secretory IgA antibodies and white blood cells. Regardless of how long breastfeeding continues, the colostrum of the first days provides an immune boost that formula cannot replicate.
Movement in the third trimester
Movement in the third trimester is both clinically important and emotionally significant. Understanding what it means — and what NHS guidance actually says — is one of the most practically useful things you can take from this page.
What normal movement looks like
By week 28 your baby has an established and individual movement pattern — their own rhythm of activity and rest, their own preferred times of day, their own responses to food, sound, and light. This pattern is the baseline against which any change should be measured.
Movement does not necessarily reduce as the baby grows and space decreases — the character of movements may change (fewer somersaults, more stretches and pushes) but the overall level of activity should remain consistent. A baby who is normally very active in the evenings and suddenly is not — that is worth reporting, regardless of what any kick-counting chart says.
What to do about reduced movement
NHS guidance is unambiguous: contact your maternity unit the same day if you notice a change in your baby's movements. Not tomorrow. Not after trying certain foods or drinks first. Same day.
You will not be wasting anyone's time. Reduced fetal movement is one of the most important clinical warnings available in late pregnancy, and the cost of assessing it — a CTG trace, usually — is low. The cost of not assessing it can be much higher.
The unfused skull: engineering for birth
One of the more counterintuitive facts of third trimester development: the bones of your baby's skull are deliberately not fused at birth. All 206 bones of the skeleton are present and largely formed by 29 weeks, but the skull is different. Its five major bone plates — two frontal, two parietal, one occipital — are connected not by rigid sutures but by fibrous tissue called fontanelles and sutures, which remain flexible throughout birth and infancy.
This is not an incompleteness in development. It is a design feature of extraordinary precision. During birth, the skull bones can slide over each other and compress, reducing the diameter of the head by up to a centimetre in some cases. Without this flexibility, vaginal birth would be considerably more difficult for the baby and considerably more traumatic for the mother. The moulding you may notice in a newborn's head — a slightly elongated or asymmetric shape — is the result of this compression and resolves within days.
The fontanelles close gradually after birth. The posterior fontanelle, at the back of the skull, closes by around three months. The anterior fontanelle — the larger, diamond-shaped soft spot at the top — closes between 12 and 18 months, by which point the brain has reached approximately 80% of its adult volume and no longer requires the accommodation that the open fontanelle provides.
Full term — and why the final weeks still matter
The term "full term" officially begins at 37 weeks. But the weeks from 37 to 40 are not developmentally equivalent, and this distinction has quietly become one of the more important findings in modern obstetrics.
Babies born at 37–38 weeks — "early term" — have measurably worse outcomes than those born at 39–40 weeks across a range of measures: respiratory problems, NICU admissions, feeding difficulties, temperature regulation, and cognitive development at school age. The differences are not dramatic for any individual baby, but they are consistent and population-level significant enough to have changed NHS policy on elective birth timing.
The developments that continue in the final weeks include:
- Brain myelination — the insulating myelin sheath on neural fibres continues developing, speeding signal transmission and supporting coordination
- Alveolar multiplication — the number of functional air sacs in the lungs continues to increase
- Antibody transfer — the final and highest-volume transfer of maternal antibodies happens in the last two to three weeks
- Fat accumulation — brown adipose tissue in particular continues building through week 40
- Gut maturation — the digestive enzymes and gut microbiome colonisation that begin at birth are better supported by a fully matured gut
A due date is a midpoint of the normal range, not a deadline. Normal gestation extends from 37 to 42 weeks. Going past 40 weeks with a well-monitored pregnancy is not a clinical failure — it is biology operating within its expected range.
Week-by-week: the key milestones
Appointments in the third trimester
The third trimester is the most appointment-dense period of the NHS antenatal schedule. Here is what to expect and when:
- 28 weeks — blood tests (full blood count, antibodies, glucose if not already done), blood pressure, urine, fundal height, presentation. Whooping cough vaccine if not already given.
- 31 weeks — blood pressure, urine, fundal height. Review of 28-week results.
- 34 weeks — information about birth preferences, what to expect in labour, infant feeding. Blood pressure, urine, fundal height.
- 36 weeks — presentation (to confirm head-down position), blood pressure, urine, fundal height. Group B Strep discussion. Birth preferences review.
- 38 weeks — blood pressure, urine, fundal height. Membrane sweep may be offered or discussed.
- 40 weeks — if not yet in labour. Membrane sweep offered. Discussion of post-dates monitoring and induction options.
- 41 weeks — if not yet in labour. Induction discussed and offered per NICE guidelines.
What you will notice in your body
The third trimester is physically the most demanding stretch of pregnancy. The changes are the result of carrying, sustaining, and preparing to birth a baby who is roughly doubling in weight over these weeks.
- Sleep disruption — almost universal from around 30 weeks, caused by the combination of the baby's movement, bladder pressure, rib pain, pelvic discomfort, and the physiological changes that already reduce deep sleep in preparation for newborn care
- Braxton Hicks contractions — increasing in frequency and sometimes intensity; the uterus rehearsing the coordinated muscular contractions of labour. They are irregular, do not increase in frequency or intensity over time, and usually subside with movement or rest
- Pelvic girdle pain (PGP) — the hormone relaxin loosens the ligaments that hold the pelvis together in preparation for birth. For some people this causes significant pain, particularly with walking, climbing stairs, or turning in bed. Referral to an obstetric physiotherapist can help significantly
- Heartburn and breathlessness — the uterus pushes up into the abdominal cavity, compressing the stomach and diaphragm. Both symptoms typically ease after the baby's head engages (drops into the pelvis), which happens in the final weeks for first-time mothers
- Increased awareness of movement — as the baby grows, every movement is felt more clearly. Many people describe an emotional quality to the third trimester movements — a sense of getting to know someone who has not yet arrived
Week-by-week pages for the third trimester
Each week from 28 to 40 has its own detailed page covering what is happening with your baby, what you might be feeling, appointments, and what to do if you have concerns.
The third trimester (weeks 28–40) is when your baby completes development and prepares for birth. The brain folds into its characteristic wrinkled structure, body weight nearly quadruples through fat accumulation, the lungs complete their maturation, maternal antibodies transfer across the placenta, and all the skills needed for the newborn period — sucking, swallowing, breathing, thermoregulation — are rehearsed and refined.
A baby is considered physiologically complete by 37 weeks — all major organ systems are functional and the baby is ready for birth. However, development continues meaningfully through weeks 37–40. Babies born at 39–40 weeks have measurably better outcomes than those born at 37–38 weeks. NHS guidance avoids elective birth before 39 weeks without a medical reason precisely because of this ongoing maturation.
A change in your baby's individual movement pattern — not a comparison to any external chart — should be reported to your maternity unit the same day. Do not wait until tomorrow, and do not rely on kick-counting apps. NHS guidance is clear: contact your unit if movements feel different from usual. Monitoring is quick, non-invasive, and never a waste of time.
The skull bones are deliberately unfused at birth — connected by flexible fibrous tissue — so they can compress and mould during birth, reducing the head diameter. This is not incomplete development; it is precisely engineered flexibility. The anterior fontanelle (soft spot) closes between 12 and 18 months, by which point the brain has reached approximately 80% of its adult volume.